Since we have about 200 trillion cells and each one performs millions of chemical reactions, the total number of chemical reactions in the human body is too huge to predict with any certainty.

Update

I'm a chiropractic student and watched a video earlier today that indicated the human body undergoes about 400 billion chemical reactions per second every second of your life.

Another Possible Answer

The estimate of cells in the human body ranges from 100-200 trillion. So if every cell did at least one chemical reaction per second, than the number of reactions must truly be higher than 400 billion. Cells perform thousands, if not millions of chemical reactions in every cell every second. I agree with the first answer that the complexity and the total number is too huge to predict or even imagine.

If we have 100-200 trillion cells in our bodies, than we have a thousand more times cells in our bodies as the number of stars in the Milky Way galaxy. If we take one step forward and ask how many atoms do we have in our bodies, that number is also about a thousand more galaxies that we know to exist in the universe.

Perhaps it is poetic to say that we are truly made of stardust, and each one of us is the product of several second generation stars that threw up enough dust into space to ultimately congeal into our solar system, with our sun as the center, and this wet rocky planet we all call home, Earth, the third rock from the sun.

Another Update

Quite unlikely that *every* cell in our body performs a chemical reaction every second. Consider for instance, that while 60% of our total volume of blood is found in the veins, only 5% is found in the capillaries where nutrient and waste products are exchanged. Considering keratinized epithelial cells, most apical layers are going to be dead so they aren't going to be reacting much and that's a pretty big part of the largest organ in our body, the skin. Assuming a person has decent amounts of calcium then osteoblast and osteclast should be in a reasonable state of homeostasis. Some cardiac tissue does have fast calcium channels but those are only open for a few 10,000ths of a second over the course of a cardiac cycle which is about 0.83 seconds on average (Guyton's). You'll find a vast number of muscles along the spinal column but the greatest numbers of muscles for proprioception are found along the vertebral columns and as such their main function is to provide an awareness of our spatial orientation.

The crux of this info comes from biochemistry, physiology and neuroanatomy and all three of my teachers for those classes come from some pretty well respected med schools (Wash U & SLU) - I'm checking the "bible" of physiology - i.e., Guytons Textbook of Medical Physiology and that book does cite 100 trillion for the total number of cells in the entire body. About 25 trillion of those are red blood cells. Of course, rbc's lack mitochondrians so metabolic pathways such as the TCA cycle would be excluded from their repertoire of activity.

Even things like a triacylglyceride (TAG) say a VLDL will have a CII polypeptide that's looking for a receptor (which it should find in an adipocyte) but after that's it's an IDL with a B100 polypeptide and that's only going to find a receptor at the liver and it does take times for various lipoproteins to circulate throughout the system.